1. Field of the Invention
The present invention relates to servo tracks written on magnetic tape to assist tape access machines in locating and positioning tape access heads on the magnetic tape and to otherwise access the magnetic tape.
2. Background Art
Magnetic tape is commonly used to store voice and data information due to its reliability, cost efficiency, and ease of use. Magnetic tape may be made more useful and cost-effective by increasing the areal density of information stored on the magnetic tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, the increase in density of data tracks requires a narrowing of the width of the data tracks, a narrowing of the spacing between data tracks, or both. As the data tracks are made narrower or are more closely spaced, positioning of the tape with respect to the tape head becomes more critical to reduce the possibility of errors introduced while reading or writing.
Tape heads generally include read elements for reading data from the magnetic tape and write elements for writing to the magnetic tape. Typically, read elements may be formed in a read module with one read element for each data track that is to be simultaneously read. Similarly, write elements are manufactured into a write module, with one write element for each data track to be simultaneously written. Thin film construction techniques are used to achieve the small geometries required to produce read elements and write elements capable of accessing densely packed data tracks. To permit read-after-write operation on tape moving in either direction over the tape head, a typical tape head may include a sandwich of one write module between two read modules.
In order to increase the accuracy of positioning the tape head relative to the tape, servo tracks or stripes may be used to provide one or more reference points. One or more servo tracks may be used depending upon the number of data tracks which are placed upon the tape, the number of tracks simultaneously accessed, and the like. Servo read elements in the read modules or write modules sense tracking patterns on the servo stripe and produce signals which are received by a control system. The control system positions the head based on the servo signals.
One type of servo pattern allowing the position of a servo read element across the width of a servo track uses two sets of low frequency transitions in each servo frame. The two sets of low frequency transitions are recorded at a relative angle to each other at a given transverse location across the servo track. Thus, a time difference between accessing transitions in the first set and accessing transitions in the second set provides an indication of the servo read element location across the width of the servo track.
A servo track may contain information in addition to fine transverse location. For example, a servo stripe number may be encoded in the servo track for coarse transverse location. A longitudinal value may be encoded in some or all servo frames to indicate position of the access head along the tape length. One method for encoding such additional information is to vary the spacing between one or more low frequency transitions in each set of transitions. For example, the second transition in each set may be moved closer to the first transition to indicate a binary one and may be spaced equally between the first and third transitions to indicate a binary zero.
There are several problems associated with varying the spacing between low frequency transitions in one or more servo frames. First, the rate of information transfer is low, with typically only one bit communicated per servo frame. Second, the technique is asymmetric, requiring complicated logic in the tape access system to correctly interpret transition spacings when reading the tape in either direction. Third, positional shifting of low frequency transitions causes peak shifting of waveforms received from the servo read element, thus changing the servo read waveforms. The change in waveform shape requires additional electronics for correct interpretation. Fourth, the low frequency pattern is typically written by a single current driver and thus cannot contain any information that varies between the servo tracks, such as a servo stripe number.
What is needed is to provide additional information in servo tracks containing fine positioning low frequency transitions that does not require modifying the low frequency transitions to convey this information.
The present invention provides a servo track write head writing low frequency transitions for fine transverse positioning with high frequency fields providing additional information.
A servo track write head for writing a plurality of servo frames as a servo track on a magnetic tape is provided. The servo track write head includes a first write gap with at least one gap section at a first angle relative to a tape direction. A second write gap has at least one gap section at a second angle, not equal to the first angle, relative to the tape direction. The first and second write gaps write low frequency transitions in each of at least a subset of servo frames. A third write gap writes high frequency transitions in each of the subset of servo frames. The third write gap may be perpendicular to the tape direction.
In an embodiment of the present invention, the servo track write head has a fourth write gap writing a timing signal on the tape. The servo track write head also includes a read element positioned to read the timing signal from the magnetic tape. This timing signal is used to determine when to write the high frequency transitions.
In another embodiment of the present invention, the servo track write head includes a read element positioned to read low frequency transitions written by at least one of the first write gap and the second write gap. The low frequency transitions read by the read element are used to determine when to write the high frequency transitions.
In still another embodiment of the present invention, the first write gap and the second write gap are constructed on a first module and the third write gap is constructed on a second module. The first module may include a C-core generating magnetic flux emitted by the first write gap and the second write gap. Alternatively, the first module may be a thin-film module defining the first and second write gaps. The second module may include a read element having a read shield forming a pole defining the third write gap. The second module may alternatively include a read element separated from poles defining the third write gap by a thin film insulating layer. A third module, defining the read element, may also be included so that the read element is separated from the high frequency write element.
In yet another embodiment of the present invention, the third write gap writes high frequency transitions forming a timing pattern that may be read to obtain timing information.
In a further embodiment of the present invention, the third write gap writes high frequency transitions forming a longitudinal position pattern that may be read to obtain position along the tape length.
In a still further embodiment of the present invention, the third write gap writes high frequency transitions forming a transverse position pattern that may be read to obtain position across the tape width.
A servo track write head for simultaneously writing a plurality servo tracks on a magnetic tape is provided. The servo track write head includes a first module having a plurality of write gap pairs for writing low frequency transitions on one of the plurality of servo tracks. Each write gap pair has a first write gap with at least one first gap section at a first angle relative to the tape direction and a second write gap with a second gap section corresponding to each first gap section, each second gap section at a second angle relative to the tape direction not equal to the first angle of the corresponding first gap section. Each first gap section and the corresponding second gap section writes low frequency transitions at the same transverse distance across a width of the tape. The servo track write head also includes a second module having a plurality of third write gaps for writing high frequency transitions on one of the servo tracks. Each third write gap corresponds to one of the plurality of write gap pairs.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.